6.5

CVSS Score

3.1

-

CVSS Score

Basic Information

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CVE-2026-5163

CVE-2026-5163: Mattermost doesn't verify channel membership when processing AI-assisted message rewrites

Mattermost versions 11.5.x <= 11.5.1 fail to verify channel membership when processing AI-assisted message rewrites which allows an authenticated attacker to read the content of threads in private channels and direct messages they do not have access to via a crafted request to the post rewrite endpoint.. Mattermost Advisory ID: MMSA-2026-00645

(GitHub Advisory)

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CVE-2026-5163

CVE-2026-5163:

6.5

CVSS Score

3.1

-

CVSS Score

Basic Information

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Technical Details

Package Name	Ecosystem	Vulnerable Versions	First Patched Version
github.com/mattermost/mattermost/server/v8	go	>= 11.5.0, < 11.5.2	11.5.2
github.com/mattermost/mattermost/server/v8	go	< 8.0.0-20260401090745-f4d1abe7e8f5	8.0.0-20260401090745-f4d1abe7e8f5
github.com/mattermost/mattermost-server	go	< 5.3.2-0.20260401090745-f4d1abe7e8f5	5.3.2-0.20260401090745-f4d1abe7e8f5

Technical Details

Vulnerability Intelligence
Miggo AI

Root Cause Analysis

The vulnerability lies in the AI-assisted message rewrite feature of Mattermost. An authenticated user could craft a request to the /posts/rewrite API endpoint, specifying a root_id of a post from a private channel or direct message they are not a member of. The backend code, specifically the buildThreadContextForRewrite function, would proceed to fetch the entire thread content associated with that root_id without first verifying if the requesting user had access to that channel. This fetched content, containing private messages, would then be passed to the AI service for processing. The patch addresses this by introducing an authorization check using GetPostIfAuthorized within the buildThreadContextForRewrite function. This ensures that before any thread content is fetched, the system verifies that the user has permission to access the post and its corresponding channel. Additionally, the calling function RewriteMessage was updated to properly handle and propagate the authorization error, preventing the rewrite process from continuing if the user lacks access.

Vulnerable functions

App.RewriteMessage

server/channels/app/post.go

The `RewriteMessage` function was modified to stop ignoring authorization errors returned from `buildThreadContextForRewrite`. Previously, it would log the error but continue execution, allowing an attacker to proceed with the rewrite request even if they didn't have access to the channel. The patch changes this behavior to return an error immediately, preventing the vulnerability.

App.buildThreadContextForRewrite

server/channels/app/post.go

This function is the root cause of the vulnerability. It was responsible for fetching the content of a thread for the AI-assisted rewrite. Before the patch, it directly fetched the thread posts using `GetPostThread` without verifying if the user had the necessary permissions to read the channel where the thread resided. The patch introduces a call to `GetPostIfAuthorized` to perform this check before any thread content is accessed, thus mitigating the information disclosure vulnerability.

Vulnerability Intelligence
Miggo AI

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WAF Protection Rules

WAF Rule

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Reasoning

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